The present invention relates to a method for producing a photographic element or more particularly to lithographic printing plates containing a particular class of light sensitive, aromatic diazonium salt condensation products, a colorant and a small amount of water insoluble polyvinyl acetal resin. Upon imagewise exposure of the element, it is developable with an aqueous developer which does not contain organic solvents.
It is known to produce polycondensation products by reacting aromatic diazonium salts, particularly diphenylamine-4-diazonium salts, with active carbonyl compounds, preferably formaldehyde. Condensation products of this type are described in U.S. Pat. No. 2,063,631. Their uses are described, e.g., in U.S. Pat. No. 3,235,383.
Such condensation products are used on a large scale in the production of printing forms. For this purpose, products for the condensation of which formaldehyde is used as the carbonyl compound have gained practically exclusive acceptance.
Although formaldehyde condensation products are still produced and used on a large scale, they have the disadvantage that their composition and properties can only be varied to a limited degree. When these products are used for the production of planographic printing forms, the oleophilic character of their light-hardening products is normally not sufficient in itself. The developed image stencil is therefore frequently reinforced by coating the stencil with a solution of an olephilic resin. Such resins may also be admixed into the image layer from the start, but it is then generally necessary to add organic solvents to the developer.
In more recent times, the problem of variability has been solved by producing mixed condensation products, in the production of which, in addition to a carbonyl compound and a diazonium salt, a second component is employed which is condensable with formaldehyde.
These products and their uses are described in U.S. Pat. No. 3,867,147. They can advantageously also be produced by condensing the diazonium salt with a second component which carries reactive methylol groups or the esters or ethers thereof, the reactive groups being introduced, in the simplest case, by reacting the second component with formaldehyde. These condensation products yield high-quality radiation-sensitive materials, in particular printing plates, and are used on a large industrial scale.
It is, however, a disadvantage of these condensation products that in the exposure of binder-free layers comprising preferred representatives of these condensates, not all of the diazonium groups are decomposed. As a result, oleophilicity is reduced. Under the action of light, the undecomposed diazonium groups may, moreover, react with the hydrophilic polymer used for preservation, to form ink-repellent reaction products.
It is true that by increasing the amount of oleophilic second component in the condensation product, condensates are obtained which are to a higher degree oleophilic, but developing these condensates is more difficult and they are less suitable for storage then the normally preferred representatives.
In column 8, lines 32 to 42 of the aforementioned U.S. patent, another possibility of variation in the preparation of the condensation products is mentioned comprising the condensing of monofunctional and bi- or polyfunctional second components with the diazonium salt, thus limiting the molecular weight of the polycondensation products. A monofunctional second component is only used in Example 23 and is there combined with formaldehyde.
In European Published Patent Application No. 61,150, a highly light-sensitive and oleophilic condensation product is obtained by condensing the second component with itself and then reacting it with a monomeric diazonium compound. In the light-sensitive layer, this condensation product is, however, also less suitable for storage and is more difficult to develop.